(#20994) Fix memorysize and memoryfree facts on AIX 

Previous versions of facter (1.6.8 at least) did not have memorysize and memoryfree facts on AIX.  With 1.7.1 the facts are there but set to 0.  

This patch uses the svmon command line utility to set those facts correctly on AIX.

svmon returns instantly, and is runnable by non-root users.
This patch was tested on AIX version 5.3, 6.1, and 7.1.